In the operation of electronic equipment, it is necessary to reduce EMI/RFI emissions to acceptable levels. The electronic keyboard used, for example, in computer workstations is an inherently difficult device to shield adequately against the emission of undesirable electromagnetic interference because of the keys themselves. Such keys represent mechanically moving components which by their very nature must protrude from the surface of the electronic component assemblies. Barring the use of an expensive and potentially unreliable flexible shield membrane, probably the best solution to the problem is to contain the electronics in a suitably shielded enclosure having appropriate-sized apertures for the keys. Such an arrangement adequately suppresses emissions from the elctronic circuits in general. However, any electric field existing between portions of the key assembly and the shielded enclosure will create an external electric field and a concomitant interference problem. External magnetic fields resulting from sizeable differential currents flowing in the key assembly may be effectively cancelled by judicious placement of incoming and outgoing circuit leads. Thus, the minimization of interference from the keyboard assembly principally involves the reduction of electrostatic effects.
The saturable-core keyboard to which the present invention is directed, utilizes the properties of a toroidal magnetic core in the presence of a permanent magnet attached to a key associated therewith. The core is wound with input and output windings. With the key in its non-actuated, undepressed state, the magnet is situated in close proximity to the core. Thus, the magnetic material of the core is saturated and its permeability is very low. Under this condition, transformer action is not present to any significant degree, and a signal applied to a primary winding on the core is either absent or greatly diminished on a secondary winding thereon. However, when a key is depressed, the magnet moves away from the core, permitting the permeability to rise to a high value and causing a large percentage of the primary signal to appear on the secondary winding. The presence of a signal at the approximate scan time is indicative of the actuation of a given key.
In a practical environment wherein small physical size and high speed operation are required, miniature ferrite toroidal cores and single turn winding loops for the respective primary and secondary windings are utilized. The rate of current rise to produce usable voltage signals from such an arrangement is rather high, for example, 1 ampere per microsecond. Also, the current rise must be sustained for a time sufficient to permit the secondary voltage pulse to be detected, that is, approximately 100 nanoseconds. In order to accomplish the foregoing, a relatively fast current driver capable of peak currents on the order of 100 milliamperes is required.
A current driver for use in the keyboard application can be implemented in a number of ways. However, those designs which offer a high degree of simplicity and low cost are preferred by manufacturers of keyboard equipment. Also, the driver design can have a profound effect on the emission of interference from the keyboard. In general, the use of standard driver integrated circuits is favored. The circuit configuration for such circuits involves the grounding of the emitter of the driving transistor to the circuit common, while the collector is coupled to the load. The drive current is limited only by the inherently high and variable transistor gain or by external parameters, such as the use of a limiting resistance or of a current source.
A present day simple and cost effective means of limiting the energy available while providing a high instantaneous current results from a capacitor discharge technique. In addition to energy limiting which protects the electronic components, the capacitor discharge scheme allows the use of a wide drive pulse and also permits the drive transistor to saturate, thereby damping any initial transient oscillations which might contribute to interference problems. A potential source of interference is present however, on termination of the input pulse. Moreover, to a first approximation, the voltage waveform existing between the scan conductors threaded through the ferrite cores and the circuit chassis, resembles a square wave pulse and is thus rich in the fundamental frequency and lower order harmonics. Also, since the pulse leading edge exhibits a very fast rise time, considerable energy also appears at the higher harmonics. The peak-to-peak amplitude of the pulse approximates the level of the supply voltage, so that the actual condition approaches the worst possible state that could exist under normal circumstances.
What is desired is a reduction in the energy available for interference due to the resultant field. In view of the foregoing statement of the nature of the interference, two courses of action may be pursued. The first involves a reduction in the width of the drive pulse; the second, reduction of the pulse amplitude. The former lowers the energy at the fundamental and lower order harmonics, but does not provide much attenuation of the higher order harmonics. The latter, on the other hand, affects all frequencies in equal fashion. The drive circuit modification of the present invention provides both of the above mentioned actions. It reduces interference to an acceptable level, while retaining the simplicity and cost effectiveness of the aforementioned integrated circuit drive and its capacitor discharge mode of operation.